Mechanical changes in living tissue correlate with pathological changes. For example, tissue viscosity, tissue stiffness (also known as elasticity) and tissue attenuation coefficients (including tissue longitudinal-wave attenuation, and tissue transverse-wave attenuation, which is also known as shear wave attenuation) are important physical parameters for clinical practice. Various means of remotely interrogating tissue mechanical properties have been developed that exploit the radiation force of an ultrasonic beam to apply force remotely to a region of tissue within the body of a subject such as a patient (acoustic radiation force, also referred to as a push pulse or push beam) to cause deformation of the tissue. Acoustic radiation force can be applied in such a way that elastic properties may be measured, either locally at the point (called focal point) of the deformation by tracking the deformation directly through the use of longitudinal-wave-based ultrasound imaging to follow the pattern of deformation, or at an adjacent region of the focal point by tracking a shear wave propagating laterally away from the deformed region (i.e. the focal point) through shear wave velocity imaging.
Interrogation by ultrasound, for purposes of medical imaging, often makes use of longitudinal waves. A longitudinal wave is characterized by back and forth movement in the direction of propagation. In the conventional tissue attenuation measurement based on a longitudinal wave, the tissue longitudinal-wave attenuation (also called tissue longitudinal attenuation) is estimated based on an ultrasound echo signal which is simultaneously impacted by both the backscatter and attenuation of tissue. Such conventional tissue longitudinal-wave attenuation measurement has the disadvantage that, since it is difficult to separate the impact of the backscatter from that of the tissue attenuation, the accuracy is limited.
An ultrasound shear (or transverse) wave, by contrast, is characterized by back and forth movement that is perpendicular to the direction of propagation. Nowadays, many commercial ultrasound scanners offer ultrasound shear wave elastography products to measure tissue shear elasticity. Tissue shear viscosity and shear wave attenuation estimation by means of shear wave elastography have not been commercialized yet. They remain scientifically active research topics as their clinical potential is emerging in certain applications. FIG. 1 illustrates measurement using shear wave according to the prior art. A push pulse (also known as push beam) 110 is transmitted toward a focal spot 130 to generate a shear wave 150 which propagates out from the focal spot in a direction, such as a lateral direction x, perpendicular to the propagation direction of the push pulse (i.e. the longitudinal direction z). One or more tracking pulses (also known as tracking beam) are transmitted, and ultrasound echo signals are received along a plurality of tracking lines (called “A-lines”) 120, 122, 124 so as to estimate, at each of a plurality of sampling locations 140, 142, 144 spaced along the lateral direction, the phase or the propagation time of the shear wave. The estimated phases or propagation time of the shear wave at the plurality of sampling locations are further used to derive the velocity value of the shear wave. The derived velocity value of the shear wave can be used to generate an ultrasound image, which process is known as radiation force impulse imaging, and/or to derive a mechanical property such as tissue viscosity or elasticity. Such conventional shear wave elastography techniques can provide tissue shear elasticity, shear viscosity and shear wave attenuation.
US2012/0089019A1 and US2011/0263978A1 are both directed to conventional shear wave elastography techniques, namely estimating tissue mechanical property on basis of its impact on the propagation of the shear wave.